v0.5.0

fix(setup): add RUNAR_HOME override for cross-platform home redirection

Tests that mutated HOME were no-ops on Windows because dirs::home_dir()
reads USERPROFILE there. Tests polluted the real user profile, leaving
stale .env and .disable-hooks files that broke subsequent tests on the
Windows CI runner.

home_dir() now checks RUNAR_HOME first, falling back to dirs::home_dir().
A shared test_support module serializes mutations via a single mutex so
parallel tests across hooks_runtime and setup don't race.